The present invention relates to a method and system for spraying a viscous material on a wafer, and more particularly to a method and system for spraying a viscous material on a wafer to increase the uniformity of the viscous material coating on a wafer.
Because of the competition, the cost of manufacturing is very critical to semiconductor manufacturing industry. Because most viscous materials, such as photoresist, SOG, and spin-on polymers, e.g. SILK and FLARE(copyright), are very expensive and most of them will be spun out of the surface of the wafer when coating, if the volume of these materials can be reduced, the manufacturing cost also will be effectively reduced. Furthermore, the coating uniformity is also very critical for a semiconductor manufacturing process. Therefore, it is a goal for semiconductor manufacturing industry to keep the coating uniformity and to reduce the volume of usage of these viscous materials simultaneously.
Take photoresist as an example. Conventionally, a pump delivers photoresist to a nozzle at a fixed pressure to spray the photoresist on a wafer. See FIG. 1. The major drawback of this prior art method is that the volume of photoresist would be too much. For example, when using TOK(copyright) IP-3100 as a coating material, if the standard deviation of the photoresist thickness coated on a wafer is 13 A, the volume is 2.5 cc.
To alleviate this drawback, another prior art method (shown in FIG. 2) was proposed. In FIG. 2, the pumping pressure is fixed in an initial predetermined period and then reduced linearly in response to the end of the initial predetermined period. Although using this method would reduce the volume of photoresist used, the coating uniformity would be affected severely. For example, when using TOK(copyright) IP-3100 as a coating material, the volume is reduced to 1.5 cc, but the standard deviation of the photoresist thickness coated on a wafer is increased to 30 xc3x85. FIG. 3 shows the standard deviation diagram in accordance with the prior art method (FIG. 2) using TOK(copyright) IP-3100 as a coating material.
Still another prior art method to reduce the volume of the photoresist used is to pre-wet the surface of the wafer with a solvent before coating for reducing the surface tension of the wafer, thereby reducing the volume of the photoresist. However, while the method reduces the volume of photoresist used, the coating uniformity cannot be improved effectively.
Hence, how to maintain the coating uniformity requirement and to reduce the volume of usage of these viscous materials simultaneously is very important for the semiconductor manufacturing industry.
When viscous materials are coated on a wafer, a nozzle sprays the materials at the center of the wafer. By spinning the wafer, the viscous materials will be delivered to the perimeter of the wafer. Because highly volatile solvents are usually added to these viscous materials before coating, the proportion of solvents in these coating materials at the center of the wafer is higher than that on the other area of the wafer. When these viscous materials are delivered by spinning, the proportion of solvents decreases because of volatility. Hence, the proportion of the viscous materials along the periphery of the wafer is higher than that at the center of the wafer. After a time period, the thickness of these materials is highest along the periphery than thickness at the center of the wafer, which forms a dish shape. When the volume of these coating materials used is large, the dishing phenomenon is not serious. However, if one wants to reduce the volume of these coating materials for cost reason, the dishing phenomenon becomes more of a concern. However, if these materials can be uniformly delivered to the periphery of the wafer, not only the volume of these coating materials can be reduced, but also coating uniformity can be effectively improved.
It is therefore an object of the present invention to provide a cost-effective method and system to obtain a good coating uniformity.
It is another object of the present invention to provide a cost-effective method and system to reduce the volume of viscous materials used when coating.
In order to overcome the shortcomings mentioned above, the present invention discloses a method and system for spraying a viscous material on a wafer, not only to keep the coating uniformity requirement but also to reduce the volume of usage of these viscous materials simultaneously.
In accordance with one embodiment of the present invention, a method for spraying a viscous material on a wafer is provided which includes the steps of: spraying the viscous material on the wafer during a first time period at a first predetermined pressure; spraying the viscous material on the wafer at a second predetermined pressure after the first time period, the second predetermined pressure being lower than the first predetermined pressure; and spraying the viscous material on the wafer during a second time period at a time-varying pressure, the time-varying pressure being increased from the second predetermined pressure to a third predetermined pressure during the second time period.
In accordance with another embodiment of the present invention, a spraying system for spraying a viscous material on a wafer is provided which includes a nozzle for spraying the viscous material on the wafer; a pump, coupled to the nozzle; and a controller coupled to the pump; the controller controlling the pump to pump the viscous material during a first time period at a first predetermined pressure; the controller being responsive to the end of the first time period for controlling the pump to deliver the viscous material during a second time period at a time-varying pressure; wherein the time-varying pressure is increased from a second predetermined pressure to a third predetermined pressure during the second time period, and the second predetermined pressure is lower than the first predetermined pressure.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.